Middle East respiratory syndrome coronavirus (MCoV) emerged in Saudi Arabia in 2012 and has been disseminated into other countries in the Middle East, North Africa and Europe. Although MERS is most probably a zoonosis, MCoV can spread person-to-person. MCoV infection causes fever, cough and pneumonia leading to respiratory failure, and the reported case fatality is ~50%. Little is known about the mechanisms of MCoV's high virulence and pathogenesis. Coronaviruses (CoVs) carry a single-stranded, positive-sense RNA genome of ~30 kb. Immediately after infection, the translation of the viral genome produces two large polyproteins that are processed into 15 or 16 mature nonstructural proteins (nsp1-nsp16), most of which are involved in viral RNA synthesis, while some have other biological functions. Nsp1 protein of various CoVs inhibits host gene expression. Viral proteins that inhibit host gene expression are often major virulence factors, and, hence, CoV nsp1 proteins most probably play a critical role in CoV pathogenesis. Indeed, mouse hepatitis virus nsp1 is a major virulence factor, and SARS-CoV (SCoV) nsp1 inhibits the production of type I interferon and interferon-stimulated genes in infected cells. Nsp1 proteins of different CoVs use divergent strategies to exert host gene expression suppression; by binding to the 40S ribosomal subunit, SCoV nsp1 inhibits mRNA translation and induces endonucleolytic mRNA cleavage, while nsp1 of transmissible gastroenteritis virus does not induce mRNA cleavage, yet suppresses translation without binding to the 40S ribosomal subunits. Our data that MCoV nsp1 inhibited translation of host mRNAs and promoted mRNA cleavage without binding to the 40S subunits and other experimental results led us to hypothesize that MCoV nsp1 suppresses host gene expression by using mechanisms that have not been described in any viral proteins. This application aims to delineate the mechanisms of MCoV nsp1-induced translation inhibition and mRNA cleavage. We will also clarify the strategy that allows robust viral gene expression in MCoV-infected cells under conditions of nsp1- induced translation inhibition. The proposed studies will provide a foundation for understanding the modulation of host gene expression by MCoV and expand our knowledge of viral pathogenicity at the molecular level.